The present application relates to an optical element and a method for manufacturing a master for producing an optical element. In particular, the present application relates to an optical element in which a plurality of structures including convex portions or concave portions are arranged on a base member surface.
In related art, regarding an optical element including a light-transmissive substrate, e.g., glass or plastic, a method in which fine, dense concave and convex (sub-wavelength structure; moth-eye) shapes are disposed on an optical element surface is mentioned as a method for reducing light due to surface reflection so as to improve a transmission characteristic. In general, in the case where a periodic concave and convex shape is disposed on an optical element surface, diffraction occurs when light pass through the concave and convex shape, and a straight-ahead component of the transmitted light is reduced significantly. However, diffraction does not occur in the case where the pitch of the concave and convex shape is smaller than the wavelength of the light to be transmitted. For example, in the case where the concave and convex shape is conical, an effective antireflection effect and excellent transmission characteristic can be obtained with respect to light with a single wavelength in accordance with the pitch, the depth, and the like of the concave and convex shape.
For example, A non-patent document by NTT Advanced Technology Corporation, “Hachouizonsei no Nai Hanshaboushitai (Mosuai) You Seikeikanagatagenban (Molding Die Master for Antireflector (Moth-eye) Exhibiting No Wavelength Dependence)”, [online], [Searched on Sep. 3, 2007], Internet <http://keytech.ntt-at.co.jp/nano/prd_0016.html>, discloses an optical element having the above-described configuration. This optical element is produced as described below. A concave and convex photoresist pattern is formed by electron beam recording on a photoresist on a Si substrate, and the Si substrate is etched while the concave and convex photoresist pattern is used as a mask. In this manner, as shown in FIG. 16, a Si master having sub-wavelength structures (pitch: about 300 nm, depth: about 400 nm) in the shape of fine tents are produced.
Regarding the Si master produced as described above, an antireflection effect can be obtained with respect to light with a wide wavelength range. Furthermore, as shown in FIG. 17, the above-described sub-wavelength structures are formed into the shapes of hexagonal lattices and, thereby, a very high performance antireflection effect (reflectance of 1% or less) can be obtained in a visible light region (refer to FIG. 18). In FIG. 18, I1 and I2 represent the reflectance of a Si flat portion and the reflectance of a pattern portion, respectively.
Subsequently, a Ni plating stamper of the resulting Si master is produced. As shown in FIG. 19, concave and convex structures which are the reverse of those of the Si master are disposed in a predetermined region on the surface of this stamper. The resulting stamper is used so as to transfer the concave and convex pattern to a transparent polycarbonate resin. In this manner, a desired optical element (duplicate substrate) is obtained. This optical element can exert a high performance antireflection effect (reflectance of 0.3% or less) as well (refer to FIG. 20). In FIG. 20, I3 and I4 represent the reflectance without pattern and the reflectance with pattern, respectively.
Regarding the optical element disclosed in the above-described non-patent document, the reflectance can be reduced to 0.3% or less. However, in recent years, further reduction of the reflectance of the optical element has been desired.
Accordingly, it is desirable to provide an optical element exhibiting further excellent antireflection characteristic and a method for manufacturing a master for producing an optical element to produce the above-described optical element.